3D printing technology is blue-printed by a computer three-dimensional design model to stack and bond special materials, such as metal powder, ceramic powder, plastic and cell tissue in a layer-wise manner, by means of a laser beam, a hot-melt nozzle etc. via software layered discretization and a numerical control molding system so as to finally mold same by superimposition to manufacture an entity product. Different from the traditional manufacturing industry of shaping and cutting raw materials by means of machining, such as mould and turn milling, so as to finally produce a product, 3D printing turns a three-dimensional entity into several two-dimensional planes for production by processing and superimposing the materials layer by layer, thereby greatly reducing the complexity of manufacturing. This digital manufacturing mode can directly produce any shape of parts from computer graphic data without needing a complicated process, a large machine tool and massive manpower, so that production-manufacturing can extend to a broader range of production crowds.
At present, molding manners of 3D printing technology are still evolving, and materials used are also varied. In various molding manners, a photo-curing method is a relatively mature manner. The photo-curing method uses the principle that photosensitive resin is cured after being irradiated by an ultraviolet laser to perform material molding by accumulation, and has the characteristics, such as high molding precision, good surface finish and high material utilization.
FIG. 1 shows a basic structure of a photo-curing 3D printing device. The 3D printing device 100 comprises a material tank 110 for accommodating photosensitive resin, an imaging system 120 for curing the photosensitive resin and a lifting platform 130 for connecting a molding tool. The imaging system 120 is located above the material tank 110, and can irradiate a light beam image to enable a layer of photosensitive resin at the liquid level of the material tank 110 to be cured. After the imaging system 120 irradiates the light beam image to enable a layer of photosensitive resin to be cured each time, the lifting platform 130 will drive the layer of molded photosensitive resin to slightly drop, and enable the photosensitive resin to be uniformly spread on a top surface of the cured tool via a blade 131 to wait for the next irradiation. The cycle repeats, and a three-dimensional tool molded by accumulation layer by layer will be obtained.
The imaging system 120 often commonly uses a laser molding technique or a digital light procession (DLP) projection technique.
The laser molding technique means using a laser scanning device to scan spot by spot. However, due to the property of the photosensitive resin, the laser power cannot be too great, otherwise, the resin will be damaged. Therefore, the moving speed of the laser is limited to a few meters to ten meters per second, causing a too slow molding speed.
The DLP projection imaging technique is realized by using a digital micromirror device (DMD) to control the reflection of light. The digital micromirror device can be considered as a mirror surface. This mirror is composed of hundreds of thousands even to billions of micromirrors. Each micromirror represents a pixel, and an image is constituted by these pixels. Each micromirror can be independently controlled to decide whether light rays are reflected to a projection lens. Finally, the whole mirror reflects the required light beam image. Due to the limitation of the resolution of a DMD chip, a defect of a relatively small molding dimension of the DLP projection imaging technique is caused, and there is a bottleneck.
A liquid crystal projection technique, as an area array image source, can theoretically project a light beam image similar to that of the DLP projection imaging technique, so as to be used for constructing the imaging system of the photo-curing 3D printing device. A liquid crystal panel contains many pixels, and each pixel can separately control a polarization direction of polarized light, and can control whether light rays of a certain pixel pass in cooperation with polarized light filters at two sides of the liquid crystal panel, and therefore, a light beam passing through a liquid crystal panel system is imaged. However, there is an obvious disadvantage for the liquid crystal panel to be used in the photo-curing 3D printing device. The reason is that the wavelength of a light source required for curing the photosensitive resin is generally below 430 nm; and light rays within the range of this wavelength may cause damage to liquid crystals in the liquid crystal panel, and the service life of the liquid crystals will be shortened. Furthermore, low transmittance of the liquid crystal panel will cause the panel endurance to be further shortened.
It is well known that a liquid crystal panel has a black mask region with a certain non-light transmitting area around each pixel for covering a control circuit of the pixel (comprising a thin film transistor, wiring, etc.). This part of mask region will reduce the light transmitting capability of an LCD panel, so as to influence the brightness and contrast of the imaging system. The ratio that the area of light transmitting regions (i.e. regions that are not covered by a mask) accounts for the total pixel area is called aperture ratio. It is assumed that the aperture ratio of the liquid crystal panel is 60%, which means that up to 40% of the area cannot transmit light and this is a great loss of the brightness of light. Meanwhile, after theses light rays are absorbed by the liquid crystal panel, excessive temperature rise in liquid crystals will be caused, thereby resulting in the ageing of and damage to the liquid crystal panel.
One way of improving the above-mentioned problems is to improve the aperture ratio as far as possible. This is admittedly helpful for reducing light loss; however, the improvement of the aperture ratio has an ultimate limit in technology, depending on a more advanced manufacturing process of a liquid crystal panel. Therefore, in the photo-curing 3D printing device, one way of making up for the shortage of transmittance is using a light source with higher brightness. However, in the case where the photo-curing 3D printing device needs relatively strong projection brightness, blindly improving the brightness of light rays that pass the liquid crystal panel exacerbates the shortening of the service life of the liquid crystals.
Table 1 below shows the comparison of service life after liquid crystals receive strong enough irradiation of light with various wavelengths in the liquid crystal projection technique.
TABLE 1Lightwavelength(nm)Endurance4100.44331.04704.2
It can be seen from Table 1 that by taking the service life of the light wavelength at 433 nm as reference 1, when the wavelength is reduced to 410 nm, the service life is significantly reduced to 0.4. In striking contrast thereto, when the wavelength is at 470 nm, the service life is significantly increased to 4.2.
Due to the above-mentioned defects in service life, no photo-curing 3D printing device applying a liquid crystal system has occurred at present.